Expressed another way, is a non-resonant antenna sometimes better for
reception than a resonant antenna?

There are several possible causes here.
Probably, on the design frequency, its antenna is probably matched to 50
ohms , for good transmission. A matched antenna does not necessarily give
the best S/N for reception, and usually a mismatch does. Its hard to
calculate.
One filing mentions the possible change in pattern, and therefore a possible
change in S/N.
Suggestion- make a number of measurements, and if it hold up, send a note
about it to QEX. Include as much info as you can, especially the SWR or
better the antenna impedance on the different frequencies.
w4mb